The Ly-6 group of murine lymphocyte antigens comprises at least five antigens. They are grouped into a multigene cluster un the basis of the close linkage of their controlling genes. To determine the exact number of these genes and to gain insight into their structure and organization are the major aims of this proposal. Ongoing work focuses on the proteins that carry the Ly-6 antigen determinants. Current results indicate that two of the five Ly-6 antigens, namely Ly 6A.2 and Ly 6E.1 constitute alleles of one and the same antigen. This conclusion is based on identity in molecular weight, electrophoretic mobility, subunit structure, and sharing of nonpolymorphic determinants that are detected by a rat monoclonal anti-Ly6 antibody. A previous report by another group that Ly 6A is a single chain protein of molecular weight 33000 may have to be revised. Using affinity chromatography, we have succeeded in purifying Ly-6 E in preparative amounts adequate for amino acid sequencing. Once sufficient sequence information has been obtained, we will construct oligo-nucleotide probes with which we hope to isolate the Ly 6E gene. The question whether Ly 6A and Ly 6E are allelic can best be answered by sequencing the corresponding genes obtained from representative strains of mice, B6 versus BALB/c. However, if sufficient quantities of Ly 6A can be obtained, sequencing of a N-terminal stretch of the Ly 6A protein may provide a satisfactory answer. (LB)